1. Field of the Invention
The present invention generally relates to a liquid crystal display device (LCD). More particularly, the present invention relates to an LCD preferably used for moving picture display.
2. Description of the Background Art
The LCDs are used for, e.g., personal computers, word processors, amusement equipments, television sets, and the like. Improvement in response characteristics of the LCDs has been studied for high-quality moving picture display.
Japanese Laid-Open Publication No. 4-288589 discloses an LCD having an increased response speed for intermediate-gray-scale display in order to reduce a residual image. In this LCD, an input image signal having its high-band components pre-enhanced is supplied to a liquid crystal display section so that the rise and fall speeds of the response are increased. Note that the “response speed” in the LCDs (liquid crystal panels) corresponds to an inverse number of the time required for the liquid crystal layer to reach an alignment state corresponding to the applied voltage (i.e., response time). The structure of a driving circuit of this LCD will be described with reference to FIG. 21.
The driving circuit of the aforementioned LCD includes an image storage circuit 61 for retaining at least one field image of an input image signal S(t), and a time-axis filter circuit 63 for detecting a variation in level of each picture element in the time-axis direction, based on the image signal retained in the storage circuit 61 and the input image signal S(t), and filtering the input image signal S(t) for high-band enhancement in the time-axis direction. The input image signal S(t) is a video signal decomposed into R (Red), G (Green) and B (Blue) signals. Since the R, G and B signals are subjected to the same processing, only one channel is shown herein.
The input image signal S(t) is retained in the image storage circuit 61 for storing an image signal of at least one field. A difference circuit 62 calculates the difference between respective picture-element signals of the input image signal S(t) and the image signal stored in the image storage circuit 61. Thus, the difference circuit 62 serves as a level variation detection circuit for detecting a variation in signal level during a single field. A difference signal Sd(t) in the time-axis direction from the difference circuit 62 is input together with the input image signal S(t) into the time-axis filter circuit 63.
The time-axis filter circuit 63 is formed from a weighting circuit 66 for weighting the difference signal Sd(t) with a weight coefficient α corresponding to the response speed, and an adder 67 for adding the weighted difference signal and the input image signal S(t) together. The time-axis filter circuit 63 is an adaptive filter circuit whose filter characteristics can be varied according to the output of the level variation detection circuit and the input level of each picture element of the input image signal. This time-axis filter circuit 63 enhances the input image signal S(t) in its high band in the time-axis direction.
The high-band enhanced signal thus obtained is converted into an alternating current (AC) signal by a polarity inversion circuit 64, and this AC signal is supplied to a liquid crystal display section 65. The liquid crystal display section 65 is an active-matrix liquid crystal display section including display electrodes (also referred to as picture-element electrodes) at the respective intersections of a plurality of data signal lines and a plurality of scanning signal lines crossing the same.
FIG. 22 is a signal waveform chart illustrating how the response characteristics are improved with this driving circuit. For simplicity of the description, it is herein assumed that the input image signal S(t) changes with a cycle period of one field, and the figure shows the case where the signal level rapidly changes in two fields. In this case, as shown in the figure, a change in the input image signal S(t) in the time-axis direction, i.e., the difference signal Sd(t), becomes positive for one field in response to the input image signal S(t) changing to positive, and becomes negative for one field in response to the input image signal S(t) changing to negative.
Basically, high-band enhancement can be achieved by adding the difference signal Sd(t) to the input image signal S(t). Actually, the relation between the respective degrees of change in the input image signal S(t) and in the transmittance depends on the response speed of the liquid crystal layer. Therefore, the weight coefficient α is determined so as to make correction within the range that does not cause any overshoot. As a result, a high-band enhanced high-band correction signal Sc(t) as shown in FIG. 22 is input to the liquid crystal display section, whereby optical response characteristics I(t) are improved as shown by the solid line over a conventional example shown by the dashed line.
In the case where the driving circuit as disclosed in the aforementioned publication is applied to a current LCD, response characteristics at a rise (a change to the display state corresponding to an increase in voltage applied to the liquid crystal layer) can be improved. However, the effect of improving the response characteristics at a fall (a change to the display state corresponding to a decrease in voltage applied to the liquid crystal layer) is relatively poor. In the LCD, a fall indicates a relaxation phenomenon that the liquid crystal molecules are restored from the orientation state corresponding to a first voltage toward that corresponding to a second voltage that is lower than the first voltage. The time required for the liquid crystal molecules to reach the orientation state corresponding to the second voltage (fall response time) mainly depends on the restoring force acting between the liquid crystal molecules. Accordingly, in the case where the voltage applied to the liquid crystal layer reduces from the first voltage to the second voltage, the fall response speed (or fall response time) of the liquid crystal layer generally does not so much depend on the magnitude of the second voltage (the difference from the first voltage). Therefore, the effect of increasing the fall response speed is poor even if the input image signal S(t) is emphasized.
It is now assumed that the lowest gray-level voltage (the lowest value of the gray-level voltage) is set to the value corresponding to the maximum transmittance in the LCD having such voltage-transmittance (V-T) characteristics as shown in FIG. 20 of the aforementioned Japanese Laid-Open Publication No. 4-288589 (corresponding to the V-T curve of 260-nm retardation in FIG. 5A of the present application). Particularly in this case, the fall response speed cannot be increased even if an overshoot voltage (a voltage lower than the lowest gray-level voltage) is applied. The reason for this is as follows: the orientation state of the liquid crystal molecules is substantially the same within a voltage region corresponding to the maximum transmittance (a flat region of the V-T curve). Therefore, the restoring force acting between the liquid crystal molecules is substantially the same whatever voltage within this region is applied.
As described above, the terms “rise” and “fall” as used in the specification correspond to a change in display state (or orientation state of the liquid crystal layer) according to an “increase” and “decrease” in voltage applied to the liquid crystal layer, respectively. A “rise”, which is a change with an increase in applied voltage, corresponds to a “reduction in brightness” in the normally white mode (hereinafter, referred to as “NW mode”) and to an “increase in brightness” in the normally black mode (hereinafter, referred to as “NB mode”). A “fall”, which is a change with a decrease in applied voltage, corresponds to an “increase in brightness” in the NW mode and to a “reduction in brightness” in the NB mode. In other words, a “fall” is associated with the relaxation phenomenon of the orientation of the liquid crystal layer (liquid crystal molecules).
Moreover, the driving method disclosed in the aforementioned Japanese Laid-Open Publication No. 4-288589 has a problem that the input image signal S(t) capable of being subjected to effective high-band enhancement is limited. More specifically, the high-band correction signal Sc(t) cannot exceed a high-band limit signal (which is herein defined as a signal having the highest voltage among the input image signals S(t) that are input to the liquid crystal display section). Therefore, the input image signal can be subjected to high-band enhancement if the high-band correction signal Sc(t)≦the high-band limit signal. However, if the high-band correction signal Sc(t)>the high-band limit signal, a correction signal enough to cause a sufficient change in transmittance cannot be input to the liquid crystal display section. Accordingly, the response speed is increased at an intermediate gray level, but the effect of improving the optical response characteristics is reduced at a higher band level (as the voltage applied to the liquid crystal display section is increased).
The present invention is made in view of the aforementioned problems, and it is an object of the present invention to provide an LCD with improved fall response characteristics. It is another object of the present invention to provide an LCD with improved response characteristics at least at a high-band level.